1. Field of the Invention
The present invention relates to an arrangement of a circuit operating according to an electric power source in which the power supply fluctuates. More specifically, it relates to a battery-power-operated circuit, for example, suitable for an infrared (IR) remote commander operating according to primary and secondary batteries, etc.
2. Description of the Related Art
For example, in devices including an IR remote commander, circuits for operation control are formed from ICs.
Digital circuits such as ICs usually incorporate a reset circuit for monitoring a supply voltage, and they are arranged to generate a reset voltage or to start or stop the operation thereof with a reset voltage supplied from the outside.
This is required in order to prevent the circuit from working at and below the lower limit of voltages at which the circuit can operate normally thereby to avoid unwanted malfunctions. Hence, the digital circuit is made inoperable at and below the voltage lower limit.
In a circuit according to an electric power source which supplies an unstable voltage, such as the case where an electric power source is a battery, i.e. a primary or secondary battery, the life voltage of the battery is set as a reset voltage.
Further, a battery has an internal resistance, the current consumption of which decreases a terminal voltage of the battery. Therefore, the larger the current is, the earlier the terminal voltage of the battery reaches a life voltage thereof.
FIG. 8 shows an example of conventional circuits in IR remote commanders.
In the exemplary circuit in FIG. 8, two batteries are used as electric power sources.
A battery voltage of 3 volts according to the battery BAT is stabilized by the capacitor C1 and used as an operating voltage of the IC 1. The battery voltage is also applied to a series circuit composed of the IR emitting diode (LED) D1 and the transistor Q1 through the resistor R1.
The IC 1 is contained as a control circuit, which carries out the control of causing the LED to output a given command signal in response to the operational input information detected by a key matrix 3.
In the remote commander, many operation keys, which allow users to operate the electronic equipment that they are to work, are prepared. The individual keys are so arranged that the operation can be detected with electrodes making up a matrix.
The IC 1 is configured as a digital circuit in which an oscillator 4 provides clocks for operation. The IC 1 generates a command signal (pulse voltage signal) in response to the operation detected by the key matrix 3 and applies the current in response to command signals (voltage pulse) to the base of the transistor Q1 through the resistor R2.
The transistor Q1 is turned ON or OFF based on the base current according to the command signal. In a period during which the transistor Q1 is turned ON, a current flows through the IR emitting diode D1, whereby infrared rays are output therefrom. Therefore, the output infrared rays form IR command signals in response to command signals generated by the IC 1.
The IC 1 incorporates a reset circuit 2. The reset circuit 2 monitors an operating voltage supplied to the power source terminal P, and stops the operation of the IC 1 when the operating voltage at the power source terminal P is equal to or lower than a reset voltage.
In the case of the circuit of FIG. 8, while the forward dropped voltage (Vf) of the LED D1 is about 1.5 volts, the battery voltage is 3 volts and as such, the current-limiting resistor R1 must be inserted in series with the LED D1.
The power consumed by the resistor R1 only changes into heat. Such power consumption is ineffective to cause the LED D1 to emit light and deteriorates the energy efficiency of the battery.
When the IC 1 has a usual operating voltage of 3 volts and a reset voltage of about 1.5 volts, the circuit of FIG. 8 is operable to the extent of a half of the battery voltage. Although this presents no problem in battery life, two batteries are required.
From this, various types of circuits which can operate with a battery and improve the energy efficiency have been proposed, but they have presented problems as described below.
FIG. 9 shows an example of a circuit, each using one battery as an electric power source. In the circuit examples to be described sequentially, the same parts as those in a circuit example already described before then are identified by the same reference character to avoid repeating the same descriptions.
In the case of the circuit of FIG. 9, a battery voltage of 1.5 volts according to the battery BAT is stabilized by the capacitor C1 and used as an operating voltage of the IC 1.
The transistor Q1 and the coil (inductor) L1 are connected in series with each other. The IR emitting diode D1 is connected in parallel with the coil L1. In this case, the anode terminal of the LED D1 is connected to the collector of the transistor Q1.
Since the LED D1 has a forward voltage drop of about 1.5 volts, the same circuit configuration as that in FIG. 8 cannot cause a sufficient current to be passed through the LED D1. Therefore, the circuit incorporating the coil L1 as shown in FIG. 9 is required.
In the case of this circuit, in a period during which the transistor Q1 is turned ON, a current flows in response to a command signal from the IC 1 in the order of: the positive electrode of the battery, coil L1, transistor Q1, and negative electrode of the battery.
Then, in a period during which the voltage of the command signal from the IC 1 becomes Low and the transistor Q1 is turned OFF, a current flowing through the coil L1 changes into a loop current flowing from the coil L1 through the LED D1 back to the coil L1. The loop current serves as a current to cause the LED D1 to emit light.
The circuit of FIG. 9 does not have a component corresponding to the current-limiting resistor R1 in FIG. 8, so that the efficiency of energy utilization of the battery is higher.
However, a voltage supplied by one battery, 1.5 volts, is to be used as a steady-state voltage in the IC 1, whereby the reset voltage of the IC 1 is limited to 0.9 to 1.0 volt for the reason of IC manufacturing. Accordingly, when the battery is drained to lower the operating voltage to about 1 volt, the reset circuit 2 stops the operation of the IC 1.
When a current flows through the LED D1, the internal resistance of the battery BAT further lowers the terminal voltage of the battery.
As the battery is drained to some extent, the voltage generated by the battery further decreases and the internal resistance increases. While the LED D1 is emitting light, the terminal voltage of the battery further decreases and thus becomes lower than the reset voltage easily. Even in the case where the terminal voltage of the battery in no-load conditions is sufficiently high, flowing a current lowers the buttery terminal voltage. Then, when the operating voltage decreases below the reset voltage, the operation of the IC 1 is stopped.
In the case of the circuit of FIG. 9 using one battery in this manner, the difference between the steady-state voltage and the reset voltage is small. On that account, battery drain hastens the occurrence of the event where the battery drain renders the IC inoperable. In addition, even when the battery voltage is somewhat higher than the reset voltage of the IC, a momentary voltage drop can actually render the IC inoperable. Therefore, at the point in time when the battery voltage has approached the reset voltage, the battery voltage reaches its operating limit, whereby the operable time provided by the battery is shortened.
Contrarily, FIG. 10 shows an example of circuits in which a momentary voltage drop is compensated to some extent.
In FIG. 10, the power source terminal P of the IC 1 is connected to the battery terminals through a time constant provided by the resistor R3 and the capacitor C2. This prevents the voltage at the electric power source terminal P of the IC 1 from lowering immediately upon a momentary drop in the terminal voltage of the battery.
On this account, the working voltage lower limit of the circuit of FIG. 10 is somewhat lower than the above-described circuit in FIG. 9. However, it is common to both the circuits in FIGS. 9 and 10 that the power source terminal voltage of the IC 1 lowers in operation.
The voltage coming from a charge accumulated by the capacitor C2 causes a reverse passing through the resistor R3 when the terminal voltage of the battery BAT lowers. However, this also decreases the voltage of the power source terminal of the IC 1.
FIG. 12 shows an example of a circuit produced by replacing the resistor R3 of the circuit of FIG. 10 with a diode D2. Use of the diode D2 can eliminates the reverse current described above.
However, the diode has a forward voltage drop, which always lowers the voltage at the power source terminal P of the IC 1 correspondingly. For example, even when a schottky-barrier diode is used as the diode D2, the amount of the voltage drop is too large to ignore in using the diode in a circuit with one battery because of its forward voltage drop of about 0.2 volt.
In addition, the voltage at the capacitor C2 cannot rise above the terminal voltage of the battery. Therefore, as the terminal voltage of the battery lowers, the voltage at the capacitor C2 decreases to that voltage gradually.
FIGS. 11A and 11B, and 13A and 13B present the results of observations for states of the voltages in the circuits of FIGS. 10 and 12, respectively.
FIGS. 11A and 13A show voltages obtained at each of the capacitors C2, i.e. voltages at the power source terminal P in the IC 1. FIGS. 11B and 13B show terminal voltages of the batteries BAT.
Incidentally, a used battery, which had been drained to some extent, was used as the battery BAT in the observations.
The current flowing through the LEDs D1 is as large as 200 to 400 mA. Therefore, the internal resistance of the battery BAT causes a voltage drop in its terminal voltage. As shown in FIGS. 11B and 13B, the minimum voltage of the battery terminal voltage in operating conditions lowers as low as about 1.0 volt in spite of the battery having a terminal voltage of about 1.4 volts in no-load conditions.
If the circuits were that shown in FIG. 9, the ICs 1 would become inoperable at this stage.
In the case of the circuit of FIG. 10, the voltage at the capacitor C2 is leveled. Therefore, the voltage at the capacitor C2 did not lower to the minimum voltage of the battery terminal and the minimum voltage thereof was 1.26 volts, as shown in FIG. 11A.
In the case of the circuit of FIG. 12, the voltage at the capacitor C2 lowers more slowly than that in the case of FIG. 10 because of no reverse current from the capacitor C2 to the battery terminal, while it also rises more slowly. When the number of times of repeated sending (IR emission) became larger, the voltage at the capacitor C2 decreased to about 1.18 volts, as shown in FIG. 13B.
In other words, in the case where the reset voltage was set to 1 volt, operating the IC with a used battery capable of supplying a voltage of 1.43 volts in no-load conditions, the voltages at the power source terminals P of the ICs 1 for the circuits of FIGS. 9, 10, and 12 were as follows. In the circuit of FIG. 9, the voltage had reached its operating limit. The circuit of FIG. 10 had a margin of about 0.26 volt. The circuit of FIG. 12 still had a margin of about 0.18 volt.
The above results show that the operable time for the circuits of FIGS. 10 and 12 is somewhat longer than that for the circuit of FIG. 9.
FIG. 14 shows an example of the characteristics of the battery terminal voltage.
In FIG. 14, the solid line (max) shows a voltage when no current flowed into the battery; the dotted line (min) shows a voltage when a large current flowed into the battery.
The case where the reset voltage of IC 1 is set to 1.0 volt is considered. In this case, if a reset can be triggered at a min voltage, the operable time of this circuit is 8 hours; if a reset can be triggered at a max voltage, the operable time is 16 hours. In the case of the circuit of FIG. 9, a reset can be triggered at a min voltage.
The broken line (mean) corresponds to voltages at the capacitors C2 in FIGS. 10 and 12. In this case, the operable time thereof is about 12 hours.
As described above, various types of remote commander circuits, in which one battery is used, have been proposed. However, any of these circuits are still inadequate in terms of effective use of the battery energy. In the case of FIG. 9, for example, no energy is wasted by a current-limiting resistor as shown in FIG. 8, whereas the reset voltage with respect to a battery voltage becomes relatively higher and thus the battery voltage reaches the operating limit much earlier. Therefore, also in this case, effective use of battery energy is not achieved. The circuits of FIGS. 10 and 12 can present an operable time longer than the circuit of FIG. 9. However, also in these circuits, the effective use of battery energy is not sufficiently achieved.
Therefore, it is an object of the invention to provide a source-voltage-operated circuit suitable for applications in the case where the power supply of an electric power source fluctuates, including a remote control commander operated with one battery. It is another object of the invention to substantially enable effective use of energy of the power supply in such source-voltage-operated circuit by: securing a voltage for an IC, which is used even when the battery voltage decreases, using a simple step-up circuit; and consequently prolonging an operable time of the source-voltage-operated circuit.